The present invention relates to a process for treating flue gas with injection of alkali solid for removal of sulfur oxides and nitrogen oxides.
An extensive amount of effluent gas is generated from the burning of various fuels. The adverse impact on human health of certain of the pollutants has only relatively recently begun to be appreciated. Specifically, sulfur dioxide (SO.sub.2) and nitrogen oxides of various forms (NO.sub.x) are believed to be especially objectionable. Although particulates such as ash and dust are also deleterious to the human body, these solid pollutants are usually easier to remove than sulfur dioxide and nitrogen oxides.
The problems of sulfur dioxide emissions from industrial plants has become more significant in recent years due to a trend away from oil and towards coal. Coal generally has a greater amount of sulfur than oil or fuels refined from oil.
The burning of coal, especially high-sulfur coals, may pose environmental problems in addition to adversely affecting human health. Specifically, if the effluent gases from the burning of relatively high-sulfur fuels are not properly treated, the sulfur dioxide released into the atmosphere causes "acid rain". Basically, the sulfur dioxide reacts to form sulfuric acid which falls with rain.
Various techniques have been used or proposed to remove sulfur dioxide and nitrogen oxides from effluent gas.
Wet scrubbing processes use an aqueous alkali solution which is sprayed into the effluent gas as it passes through a chamber or tower. Relatively large quantites of water are used in the injected slurry in order to saturate the effluent gases. In addition to the disadvantage of requiring large amounts of water, the wet scrubbing processes generate a large amount of waste product solutions which are hard to dispose of without causing water pollution. Further, scaling or solidification of the reaction products occurs on various parts of the wet scrubbing system causing high maintenance costs.
In contrast to the wet scrubbing or washing processes, a number of dry scrubbing processes such as spray drying have been used. The spray drying process is superficially similar to wet scrubbing processes in that water is used to inject an alkali reagent such as lime or limestone into the stream of effluent gases. However, unlike the wet scrubbing or washing processes, the spray drying process uses a relatively small quantity of water which will evaporate after it has carried the reagent into a chamber through which the effluent gas passes. From the chamber the effluent gases pass into a particulate collection means such as a fabric filter or electrostatic precipitator. The particulate collection means removes the solid products of the reaction between the reagent and the pollutant such as sulfur dioxide.
Although the spray drying process has been useful in removing pollutants without being subject to the disadvantages of the wet scrubbing or washing processes, the spray drying process has other disadvantages. Specifically, the spray drying process generally requires a higher stoichiometric ratio of reagent (usually calcium from lime or limestone) to sulfur oxides than is the case in the wet scrubbing processes. Typically, in order to achieve adequate removal of sulfur dioxide from effluent gases of high sulfur content coal the stoichiometric ratio of reagent (calcium) content to sulfur oxides must be over 2 to 1. However, the solids content of the injected slurry in a spray drying process is limited to less than 30%, preferably under 25%, to avoid damaging the pump which is used to inject the slurry into the spray dryer. Accordingly, one can not raise the stoichiometric ratio of reagent to sulfur oxides unless the water content is increased. Yet the water content must be limited in order to keep the process dry and avoid the disadvantages of the washing processes discussed above. These design tradeoffs have restricted the use of the spray drying processes to limited applications.
Pollutants have additionally been removed from effluent gases by the use of an ionizing radiation. Such systems use electron beam, ultraviolet light, gamma radiation, or the like to ionize sulfur dioxide and nitrogen oxides in the effluent gases. Although the reaction mechanism for the oxidation of sulfur dioxide and nitrogen oxides using this technique is very complex and not fully understood, the ionization caused by electron beam or other ionizing radiation converts the sulfur dioxide and nitrogen oxides to acid mist at low temperatures and/or solid particles at higher temperatures.
Unfortunately, the ionizing radiation method usually requires high dosages (two to eight megarads) to satisfactorally remove the pollutants. Additionally, the acid mist has the tendency to corrode the electron beam or other irradiation chamber.
An adaptation of the ionizing radiation process as disclosed in U.S. Pat. No. 3,869,362 of Machi et al., issued on Mar. 4, 1975, may use a preliminary treatment step such as washing the effluent gases with an acqueous alkali solution in a tower prior to radiating the gases.
U.S. Pat. No. 4,372,832 issued to John R. Bush on Feb. 8, 1983, assigned to the assignee of the present invention, and entitled "Pollution Control by Spray Dryer and Electron Beam Treatment" discloses a method for treating effluent gases by use of a spray dryer followed by electron beam radiation. That patent, which is hereby incorporated by reference, introduces a reagent such as lime with water into the spray dryer which is located between an air preheater (downstream from the boiler) and the electrom beam reactor. Particulate collection occurs after the electron beam reactor.
Further, the process for sulfur dioxide removal from fossil fuel combustion where limestone is injected into the combustion zone of a boiler has been investigated and reported in the literature for about 10 to 15 years. This boiler injection process, often called LIMB, has demonstrated sulfur dioxide removal of 50% in flue gas emission where limestone stoichiometries (ratio of limestone reagent to sulfur dioxide) of 200 to 300% are employed.
Such excess limestone injection has resulted in high levels of particulate emission to create a greater demand on the particulate collection or fly ash removal unit of the system. In other words, the injection of excess limestone greatly increases the amount of particulates in the effluent gas. Existing boiler systems have limited capacity in particulate removal as dictated by the original system design. Accordingly, conversion of an in-place boiler system to the LIMB desulfurization causes excess emissions of particles. However, corrective measures for particulate removal can be overcome by adding capacity to collecting apparatus such as electrostatic precipitators or fabric filters.
As an alternative to the expensive measures of increasing particulate removal capacity the present inventors have developed a method of reduced temperature operation for particulate collection wherein limestone is injected into the boiler in an amount at least sufficient to react with the sulfur trioxide. Thereafter, the gas stream is cooled to a temperature below 250.degree. F. and then subjected to particle collection. That system is disclosed in the present inventors' U.S. patent application Ser. No. 599,792 filed on Apr. 13, 1984 and entitled "Method for Reduced Temperature Operation of Flue Gas Collectors." That application is a C-I-P of U.S. patent application Ser. No. 520,620 filed on Aug. 5, 1983 by the present inventors and having the same title as U.S. patent application Ser. No. 599,792. Both of these applications, which are assigned to the assignee of the present application, are hereby incorporated by reference.
A modification of the boiler injection of limestone has been the enhancement of sulfur dioxide removal by use of a spray dryer.
U.S. patent application Ser. No. 575,651 entitled "Limestone Injection Multiburning (LIMB) Post Combustion SO.sub.2 Removal System" filed by the present inventors on Jan. 31, 1984 as a continuation-in-part of the above discussed application Ser. No. 520,620 discloses a limestone injection process including controlling of the particles size of injected limestone or hydrated lime, humidifying the combustion gases, and increasing reaction time between humidification and particle collection. That application, which is assigned to the assignee of the present application, is hereby incorporated by reference.